Utilizing the marine mollusc Aplysia as a simple test system, a quantitative analysis of the neural organization underlying three elementary types of behavior will be performed. The first behavior, the defensive gill-withdrawal reflex, is an example of a simple graded reflex, the second behavior, the release of ink, is an example of an elementary fixed act, while the third behavior, spontaneous respiratory pumping, is an example of a simple fixed-action pattern. This proposal is an attempt to extend previous studies to a more quantitative level by determining to what extent a quantitative knowledge of the individual neural elements can account for each behavior. For the gill-withdrawal reflex the relationship between the tactile stimulus to the skin and the magnitude and time course of the reflex mediated gill contractions will be analyzed by obtaining transfer functions of the input-output characteristics of the individual neuronal and motor elements. For inking behavior it is known that many of its features are correlated with an all-or-none accelerating burst in the ink motor neurons which is delayed relative to the the onset of the stimulus. Using voltagle clamp techniques, a Hodgkin-Huxley type analysis of the ionic conductance changes which underlie the delay and the accelerating burst activity will be performed. For spontaneous respiratory pumping a further examination of the underlying neural circuit and a quantitative analysis of the mechanisms underlying the generation of the spontaneous burst which initiates the behavior will be undertaken. The relationships between the burst generating cells and the interneurons and motorneurons mediating the behavior will then be examined. For all three behaviors mathematical formulations will be developed and simulated on a digital computer to determine how well the neural processes account for the behavior.